This invention relates to 2-(3',4'-difluorophenyl)-1,3-dioxane derivatives, and more particularly to novel liquid crystal compositions including 2-(3',4'-difluorophenyl)-1,3-dioxane derivatives suitable for use in electro-optical displays.
Liquid crystal display devices utilize electro-optical effects possessed by liquid crystals. The liquid crystal materials used in these devices have a nematic phase, a cholesteric phase and a smectic phase. The most widely used display mode uses liquid crystal materials in the nematic phase and includes the dynamic scattering type (DSM), guest-host type (G-H), twisted nematic type (TN), super-twisted nematic type (STN), super-twisted birefringence type (SBE) modes and the like. The driving systems used for these liquid crystal display devices include the static driving system, time-sharing driving system (simple matrix driving system), active matrix driving system, two frequency driving system and the like.
Liquid crystal display devices have several advantages, particularly liquid crystal display devices using liquid crystal materials in the twisted nematic or super twisted nematic modes or the like. The devices are small in size and can be made thin and the devices can be driven at low voltage with low power consumption. The liquid crystal material is a light receiving element so that when a liquid crystal display is viewed over a long time, eye strain does not occur.
In view of these benefits, liquid crystal display technology has been applied to watches, cameras, electronic counters, audio equipment, automobile dashboard indicators, telephone equipment, measuring devices and the like. More particularly, liquid crystal display devices have also been utilized recently in personal computers and word processor displays and in other devices including displays which require high resolution and many pixels, including televisions and the like. Thus, liquid crystal display devices continue to attract attention as potentially replacing cathode ray tubes. As a result, liquid crystal display devices are in a wide variety of areas, and it is likely that their use will be broadened further.
For practical use, liquid crystal compositions must possess the following characteristics:
1. The liquid crystal materials must be colorless and thermally, optically, electrically and chemically stable;
2. Have a wide nematic temperature range;
3. Have a low threshold voltage (V.sub.th);
4. The temperature dependency of threshold voltage be small;
5. A wide visual angle; and
6. Have a rapid electro-optical response speed.
Many liquid crystal materials possess one of the above-desired properties, however, no single compound satisfies at least two of the above-desired properties. Thus, liquid crystal compositions are formed of several different liquid crystal compounds or liquid crystal compositions are obtained by mixing liquid crystal compounds with pseudo liquid crystal compounds to obtain the desired properties. The pseudo liquid crystal compounds are compounds resembling liquid crystal compounds in their molecular formulas, but fail to manifest liquid crystal phases.
In particular, since the integrated circuit (IC) driving the liquid crystal display device has limited resistance to voltage, when a liquid crystal display device is operated with a power source or is a large scale time-sharing device, it is desirable to decrease the driving voltage of the liquid crystal display device. In order to decrease the driving voltage of a liquid crystal display device, it is necessary to decrease the threshold voltage (V.sub.th) as much as possible, since V.sub.th represents the driving voltage. Generally, V.sub.th is reduced by including a compound having a large positive dielectric constant anisotropy (.DELTA..epsilon.). These compound include, for example: ##STR2## wherein R is a linear alkyl or alkoxy group.
These conventional compounds, however, also have large refractive index anisotropy (.DELTA.n). The visual angle is dependent upon the pre-tilt of the liquid crystal molecule and the visual angle range widens as .DELTA.n decreases and narrows as .DELTA.n increases. Thus, these conventional compounds have a narrow visual angle and narrow the visual angle of a liquid crystal display device in which they are included and are not completely satisfactory.
Accordingly, it is desirable to provide an improved liquid crystal material for use in a nematic liquid crystal composition having a low threshold voltage and a small refractive index anisotropy.